The manufacture of shoes by the method of attaching a sole to an upper by direct injection molding is in widespread use around the world. Improvements in shoe manufacturing machinery and the materials from which shoes are made have greatly expanded the applications of this method. The manufacture of shoes by this method has allowed for the expansion of product lines from very basic styles to shoes for almost all needs due to its high efficiency and low cost.
One manner whereby shoes have been made by injection molding is commonly referred to in the art as "string lasting". In this procedure, a lasting string is attached to an "upper". The upper is a pre-cut material which forms the upper portion of the finished shoe (minus the sole). The upper has a toe portion at the front, a heel portion at the back, and sides. The lasting string is stitched around the entire lower border of an upper (the area of the upper to which the sole is attached) within a wrapping stitch. The lasting string is thereby attached to the upper, but is free to move around the border. The string is stitched such that it makes one and one-half turns around the lower border of the upper. Approximately 10 inches of the string are left at each end without stitching, to catch and pull.
The stitched upper is then placed on a last. The last is a casting which is shaped similarly to a human foot and lower leg and is part of the shoe injection mold. Similarly to the human foot, the last has a toe area in the forepart, a heel in the back, shanks in both sides between heel and toe, and a leg portion, and will be referred as such for purposes of the present disclosure. The bottom of the last, corresponding to the bottom of a human foot, will be described as inferior surface.
After the stitched upper is placed on the last, the free ends of the string are pulled strongly. The string runs inside the wrapping stitching, shortening its course while pulling the upper downward. The upper is stretched tightly against the last by this action, such that the lower border of the upper is stretched tightly against the inferior surface of the last. The end of the lasting string are tied tightly. The remaining string ends are cut away. Thus, the upper becomes "lasted", or in other words, the last becomes "dressed" with the lasted upper.
Next, the last is placed in injection position in the mold. In addition to the last, the mold also includes two sides and a bottom. When mold is shut, the sides, bottom and the dressed last (placed at the top) fit perfectly together, forming the sole cavity for injection and leaving no openings through which melted plastic can leak.
The injection molding machine includes two main functional parts, an injector and a mold carrier table. The injector has a cylinder with a rotating screw inside. Injection material is melted and pushed through the cylinder at high pressure, and passes through a nozzle at an end thereof that will make contact with the injection mold at its injection point. The mold carrier table is furnished with mold carriers where molds, according style and size needed, have been previously placed and attached by means of screws.
After the last is placed in injection position, the mold is closed and hot melted plastic, rubber or the like is injected into the mold cavity through an injection point. The injection point is an opening disposed at a point along the contact line. The melted plastic completely fills the interior cavity of the mold where the sole of the shoe is to be formed. Once the mold-filling material sufficient cools, it becomes solid and strongly attached to the upper along its lower border. The mold is then opened and the injected shoe is removed for finishing steps such as cutting away plastic leakings, inserting an insole, attaching labels, and/or laces. The finished shoe is then checked for imperfections and packaged.
Although the above mentioned procedure is efficient for producing shoes whose uppers are designed to cover the foot in continuous fashion all around, it becomes problematic if used to make shoe styles having an open heel, toe or sides. Such styles are characterized by uppers having an absence of upper material in some of those areas, and an empty space is left at the lower part of the upper. In order to limit the superior border of the injection cavity (corresponding to the superior border of the injected sole), a very precise adjustment of mold is needed for filling empty space left by the lack of upper material between mold sides and last, in the open areas of the upper. Also, to obtain proper and reproducible results a very precise position of the upper on the last in proximity to the lower border of the upper is required (where the sides of the mold will press against the last when mold is shut). It is at this location where the material comprising the upper is normally present but is missing in these styles. If the position of the mold is not precisely correct in this location, a defective shoe may result. First, an imprecise position may result in excess pressure on the upper that could damage it and a defective shoe could result. Second, the lack of upper in such locations will leave room between the sides of the mold and the last. During injection molding, hot plastic may leak through, resulting in an improperly shaped and defective shoe.
It is also difficult to provide the required position in a vertical direction for the upper when injecting an open heel shoe because, when pulling the lasting string, the upper will be drawn downward disproportionately at the area immediately before the open area of the upper, as there is unequal resistance due to the lack of material at these locations. This results in two problems. The first problem is that the lower border of the upper, which is intended to tightly contact the inferior surface of the last, may not do so at all points of contact. The mold-filling material may leak through any openings between the last and upper, (i.e., at the open areas of the upper) resulting in an incomplete and defective sole. Second, excessive downward movement of the upper on the last may result in a wide-mouthed opening of the shoe (at the opening for inserting wear's foot), resulting in a shoe with poor fit and appearance.
Also, in the case of an open-heeled upper, it is very difficult to attach the stitching string for lasting from the back end of one side of the upper to the beginning of the opposite side.